Law enforcement agencies often are confronted with hostage situations where armed intruders are barricaded inside a building. Officers on the scene generally have no means for determining the number and position of persons within the building, and are thus hampered in their efforts to resolve the situation. Similarly, law enforcement personnel planning a surprise raid on an armed compound would also greatly benefit from information related to the number and position of persons within. Such situational awareness decreases the amount of risk faced by the entering law enforcement personnel by decreasing the amount of unknowns. Furthermore, such a system would be of great use to rescue agency attempting to find survivors in cave-ins or collapsed buildings.
Prior attempts to provide law enforcement and rescue personnel with a priori knowledge of the occupants of a structure include acoustic, optical and infra-red (IR) detection systems. The acoustic solution is simply to have a very sensitive listening device (i.e. microphone), or array of them, and listen to determine if there were any noises coming from the room. However, without an array of directional devices, it is impossible to determine the location of the targets generating the sound. Furthermore, moving targets may not make enough sound to be detected.
The optical solution is to somehow, view the interior of the structure through a window, or to find a crack in the structure through which to view the interior, or actually drill a hole so that a camera of some sort could be inserted and the room surveilled. The drawbacks of this solution are that it takes time to find a crack or drill a hole and it is noisy to do so. Thus, in a hostage or raid situation, the law enforcement personnel could lose the tactical advantage of surprise by virtue of lack of stealth. Additionally, view through a window or crack may only provide a limited field of view, and so, parts of the room may be hidden. Moreover, if the room is smoke filled then this solution is ineffective. Finally, the IR solution is basically a thermal mapping solution. However this cannot be implemented as a through wall device, one must have a direct view of the room. Furthermore, for obvious reasons IR devices are rendered ineffective in fire-fighting scenarios.
On the other hand, ultra wideband (UWB) radars exhibit many desirable features that would be advantageous in those sorts of environments, such as high range resolution, low processing sidelobes, excellent clutter rejection capability, and the ability to scan distinct range windows. Additionally, the technique of time modulated UWB (TM-UWB) adds decreased range ambiguities and increased resistance to spoofing or interference. Impulse radar can operate on wavelengths capable of penetrating typical non-metallic construction material. These advantages make impulse radar particularly beneficial in short range, high clutter environments. Thus, impulse radars have beneficial applicability in environments where vision is obscured by obstacles such as walls, rubble, or smoke, and fire. Various embodiments of impulse radar have been described in co-owned U.S. Pat. Nos. 4,743,906, issued to Fullerton, May 10, 1988; 4,813,057, issued to Fullerton, Mar. 14, 1989; and 5,363,108, issued to Fullerton, Nov. 8, 1994, all of which are incorporated herein by reference. Moreover, arrays of such radars have been developed for such uses as high resolution detection and intruder alert systems, as described in co-owned U.S. Pat. Nos. 6,218,979B1, issued to Barnes, et al Apr. 17, 2001; and 6,177,903, issued to Fullerton, et al Jan. 23, 2001, respectively, both of which are incorporated herein by reference. These systems benefit from being low-power, non-interfering, and yet capable of scanning through typical, non-metallic building material.
However, as indicated in the described patents, those implementations comprise two or more radar systems making them not easily transportable. The above-described scenarios benefit from ease of transport and stealth. Recent advances in ultra wideband radio technology have enabled the development of radar platforms that allow a single operator to detect and monitor targets through walls, rubble or other material.
A need, therefore, exists for a system that allows detection of moving targets through walls or other non-metallic building material, but capable of transport and operation by one user. Such a system would obviously include the capability to display target information, such as range and azimuth, to the user.
One of the difficulties of developing such devices is associated with the fact the received signals are in alternating current (AC). This means there is not a single maximum that indicates the location of the object or target. This gets even more complicated as the pulses"" number of lobes increases. Whether the target in moving near the radar or the radar is moving near the target, a specific range cell will experience nulls that are approximately periodic as the target moves through it even though it is desired to have a continuous detection. For imaging, the AC effects manifest themselves by giving the target has a xe2x80x9cbeehivexe2x80x9d look caused by periodic peaks and troughs in the image. These bands from the target response are distracting and can cause misinterpretation, particularly when multiple targets are present. If an entire time domain waveform has been collected, the envelope of the target""s response can be used to eliminate these misleading drops in the signal.
One such method to do this is a square law detector, or an envelope detector using a rectifier followed by a low pass filter. The disadvantage of such a techniques is that they require the system to sample and collect an entire waveform which requires a programmable time delay. Moreover, they introduce signal processing requirements in extremely cost sensitive systems such as proximity detectors. The driving cost requirements make it desirable to minimize the number of range cells used to make a detection decision and to minimize the required calculations. For imaging, such as in back projection techniques, a square law detector can be used along radials from the radar to produce a envelop of the back projected image. However, this can be computational intensive, an extreme problem when trying to perform in real time with inexpensive processors. This type of problem has significantly limited the availability of such radars in the commercial market because the processing requirements render such systems economically infeasible.
Thus, a beneficial method of detection and image processing of UWB radar signals is needed.
The present invention is directed to a method of detection and imaging in UWB radars that satisfies this need. The method comprises receiving an impulse waveform reflected from an object and creating an envelope by squaring that waveform, then delaying the waveform by a time interval and squaring the delayed waveform, and summing both squares. The interval is equal to the time between the occurrence of the greatest magnitude of energy displacement, either positive or negative, and an adjacent zero energy value. The interval is herein referred to as the xe2x80x9cpeak-to-zeroxe2x80x9d interval or the PZD interval. If the envelope is to be defined in terms of voltage, the root of the sum of the squares may be found.
The measurement of the interval may be based upon a permanently stored value derived from factory or pre-use calibration testing. Alternatively, the interval may be determined in real time by measuring the interval from the reflected waveform.
A further embodiment includes the step of storing the reflected waveform, and further storing the reflected waveform by sampling the waveform at a rate less than the Nyquist rate.
A further embodiment of the present invention is beneficially employed in back-projection imaging techniques whereby, in a radar device each of a plurality of waveforms is sampled at a first sampling point. The values of these samples are summed, and this sum is squared. The waveforms are delayed by the PZD interval and samples are taken at corresponding sampling points. These sampling points correspond to image pixels. The values of the samples from the delayed waveforms are likewise summed and squared. The two squared values are added together. Again, if the image envelope, thus defined, is to be represented in terms of voltage, the root of total may be found.